It is very useful to evaluate quantitatively a motion (systole/diastole) of parts, such as a heart, when getting to know its function. For example, it is known that a wall-motion is changed in part, because of shortage of blood flow supplied from a coronary artery in ischemic heart disease. About the quantitative evaluation procedure of the partial wall motion, several methods, such as Tissue Doppler method or Tracking method of brightness of B mode image, have been proposed.
In the meanwhile, in order to know a status of the blood flow to cardiac muscle, a method for imaging a blood flow inside the cardiac muscle is clinically used. In this method, strong ultrasonic scatter characteristics in response to the contrast agent injected into an object is used.
In the method for imaging the contrast agent, there are mainly a High Mechanical Index (Hi-MI) method which uses a nonlinear signal generated from the contrast agent collapsed by a relatively high power ultrasound irradiation, and a Low-MI method which uses a signal generated from the contrast agent which is not collapsed by a relatively low power ultrasound irradiation. In the both methods, a signal from the tissue is reduced, and a signal from contrast agent, namely from the blood flow, is efficiently obtained.
For instance, Japanese Patent Disclosure (Kokai) No. 8-182680 discloses an ultrasound imaging apparatus for obtaining a contrast image using a nonlinear signal, and in order to separate a contrast agent signal and a tissue signal, a tissue Doppler image is obtained from a fundamental wave signal, and the contrast image and the tissue Doppler image are superimposed.
According to the method disclosed in the Japanese Patent Disclosure (Kokai) No. 8-182680, tissue functional information and blood flow information are obtained simultaneously. It is desired that the method contributes to a diagnosis when the blood flows but the wall does not move, which is so-called as a hibernation/swoon cardiac muscle.
In Japanese Patent Disclosure (Kokai) No. 8-182680, the tissue signal is obtained from the fundamental wave, however the following various problems may exist in actual situation when the contrast agent is injected to the patient.
First, since bubbles in the contrast agent are destroyed at the time of Hi-MI, wide band signals occur and phases of received signals become random. In such a situation, a speed also becomes random based on the signal detected in the Doppler method, and therefore, even if only the fundamental signal is used, an actual speed of the tissue is not assumed correctly.
Second, it is difficult to use Harmonic TDI (Harmonic Tissue Doppler Imaging) which is hardly influenced of a fixed noise and improves accuracy of the presumption of the speed. Especially, when the tissue signal is referred from the fundamental signal or the Low-MI is used, the non-linear signal is reduced.
Third, 1.5HI (Harmonic Imaging) is a contrast image method where only a signal from a bubble, which frequency is 1.5 times of the fundamental wave frequency, is efficiently extracted, not from a tissue Doppler. The tissue signal is canceled, and it is difficult to perform a tracking using the tissue information.